Ok, assuming you have vars setup like...

Desk1Paper "c:\somepath\somewall_1.bmp"
Desk2Paper "c:\somepath\somewall_2.bmp"

Try this as option #1.

VWMOnSwitchTo !wallpaper "^@#d^Desk^@dc^Paper^@#d^"

--

And this as option #2.

vwmOnSwitchTo ^@#d^VWMOnSwitchTo^@dc^^@#d^

vwmOnSwitchTo1 "!wallpaper '$Desk1Paper$'"
vwmOnSwitchTo2 "!wallpaper '$Desk2Paper$'"

(Note the double and single quotes.)